Maximize methane recovery from sludge anaerobic digestion by combining an optimal wet air oxidation process

Abstract Anaerobic digestion (AD) is widely used to recover energy from sewage sludge, but much digested sludge remains. In this study, wet-air oxidation (WAO) was used to decompose digested sludge for further methanogenesis via AD. The results showed that carbohydrate and protein in digested sludge were degraded to acetic acid and other volatile fatty acids during WAO, but residual humic acids and their derivatives with molecular weights less than 300 Da became the main refractory substances. They accounted for approximately 10% of total chemical oxygen demand in raw sludge. After WAO, the treated sludge was dewatered, and the filtrate was returned to AD. In the integrated AD and WAO system, the peak removal rate of sludge organic matter was 83.3% at 240 °C, but the conversion rate to methane was only 42.7%, even when removed ammonia in advance. By contrast, the organic removal rate was only 71.0% in the integrated system with WAO at 180 °C, but the conversion rate to methane reached 53.3%. Moreover, the relatively low concentration of ammonia had no significant effect on AD. In comparison with only AD, the integrated system with WAO at 180 °C harvested 51.4% more methane.